If i had to pick one, I'd say Lenalee. I'm not sure you guys remember, but when the first lvl 4 appeared and she got her innocence back, Kamui commented on how she was much stronger, fighting on par with a lvl 4.

But, I think they both may have a piece of the heart. Frankly, I think its obvious. After all, their innocences did things no one thought was possible. (Allen's becoming part of his heart, Lenalee's fusing with her blood)

umm well in the end of anime, how innocence merge with leenalee's blood and form the boots i think may be it's leenalee but on the otherside how sword formed from Allen's arm it might be Allen but it might be none of them also

I say both because why didn't Allen's Innocence get destroyed by Tyki while in China? I think they both share a half, making destroying of the Heart almost impossible (if destroyed at the same time it could be possible) sort of like the Innocence's defensive act against the Earl and co.

Oohh I like that idea.. xD hehe.
But :X If there was a "None of the above" button o.o I'd choose that. xD but I think more of Allen because well o_o technically Allen's innocence is like stuck on his heart xD Okay o_o maybe "stuck" is not a good word to word that..xD uhmm o.o filled it's gap And o-o yea, o_o but Allen's innocence is also getting stronger too. But same goes with Lenalee @.@" Eeep! Well I like chikarubunny's answer too ^_^" most reasonable >:D

Out of those two I have to vote for Lenalee but I sort of am more inclined to think that it will belong to someone not yet seen perhaps since it is still hiding itself. That and the writer seems to be introducing more characters so it is not completely inconceivable that we have yet to meet the holder of the heart of the Innocence!